1. Field of the Invention
This invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material comprising a support, an undercoat layer formed on the support, comprising spherical-void plastic particles having an average particle diameter of 2 to 20 .mu.m and a voidage of 80% or more, and a thermosensitive coloring layer formed on the undercoat layer, comprising a leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto.
2. Discussion of Background
Recently, various information recording materials of a non-environmental-pollution type, capable of nursing resources and economizing energy, have been developed and put to practical use for the purpose of dealing with a great variety of information. In particular, thermosensitive recording materials have been widely utilized in various fields, for instance, for use with terminal printers for computers and calculators, recorders for medical measurement instruments, low- and high-speed facsimile apparatus, automatic ticket vending apparatus, copying machines, and label printing machines for the POS system, because of the following advantages thereof:
(1) images can be readily recorded on a thermosensitive recording material by simply applying heat thereto without employing a complicated development process;
(2) a relatively simple and .mu.mall-sized apparatus is available for preparing a thermosensitive recording material, handling of the recording material is easy, and the maintenance cost of the same is inexpensive; and
(3) in the case where paper, which is not costly as compared with other materials, is used as a support, a thermosensitive recording material with a plain paper-like touch is obtainable.
In general, the thermosensitive recording material is prepared by applying a coating liquid for forming a thermosensitive coloring layer, which contains a coloring component for color formation by application of heat, to the surface of a sheet of plain paper or synthetic paper, or a plastic film, and then dried. Images are recorded on the thus prepared recording material with application of heat by a thermal pen or thermal head. Such thermosensitive recording materials are disclosed, for instance, in Japanese Patent Publications 43-4160 and 45-14039. These conventional recording materials, however, sluggishly respond to thermal energy, so that images with high density are not obtainable when high-speed recording is performed.
In order to overcome the above shortcomings, it has been proposed to incorporate the following compounds into the conventional thermosensitive recording materials: nitrogen-containing compounds such as acetamide, stearamide, m-nitroaniline and dinitrile phthalate (Japanese Laid-Open Patent Application 49-38424); acetoacetic anilide (Japanese Laid-Open Patent Application 52-106746); N,N-diphenylamine derivatives, benzamide derivatives and carbazole derivatives (Japanese Laid-Open Patent Application 53-11036); alkylated biphenyl and biphenyl alkane (Japanese Laid-Open Patent Application 53-39139); and p-hydroxy benzoate derivatives (Japanese Laid-Open Patent Application 56-144193). However, satisfactory recording materials cannot be obtained by incorporating any of the above compounds.
Japanese Laid-Open Patent Application 56-164890 discloses that the thermal sensitivity of a thermosensitive recording material can be improved by using an amorphous leuco dye, which is obtained by converting the corresponding crystalline leuco dye to the amorphous leuco dye, and has a lower melting point than that of the corresponding crystalline leuco dye. Such amorphous leuco dyes, however, are highly reactive because of their activated surfaces, so that a coating liquid for forming a thermosensitive coloring layer containing such an amorphous leuco dye causes a so-called liquid fogging, or the thermosensitive recording material containing the same has the problem that the fogging of the background occurs, which reduces the whiteness of the background.
It is required that a thermosensitive recording material have an excellent dot reproduction property to obtain clear images with high density. In other words, it is required that a thermal head touch a thermosensitive coloring layer as closely and smoothly as possible, the thermal conduction from the thermal head to the thermosensitive coloring layer be performed as quickly as possible, and each dot be formed on a thermosensitive coloring layer in the shape faithful to the shape of each heating element of the thermal head. However, the thermal conductivity of the conventional thermosensitive recording materials is so low that only several percent of heat is conducted from the thermal head to the thermosensitive coloring layer.
Several proposals have been made for improving the surface smoothness of the thermosensitive coloring layer so that the thermal head smoothly touches the thermosensitive coloring layer. Japanese Patent Publication No. 52-20142 discloses a thermosensitive recording material comprising a thermosensitive coloring layer which is surface-treated to have a surface smoothness of 200 to 1000 sec. in terms of Bekk's smoothness. On the other hand, in Japanese Laid-Open Patent Application No. 54-115255, it is said that the thermosensitive coloring layer having the surface smoothness of 200 to 1000 sec. in terms of Bekk's smoothness is capable of performing image recording only with a thermal pulse of about 5 to 6 msec., so that it is necessary that the thermosensitive coloring layer be surface-treated to have a surface smoothness of 1100 sec. or more in terms of Bekk's smoothness for high-speed recording of 1 msec. or less. However, when the thermosensitive recording material comprising the thermosensitive coloring layer is subjected to the surface treatment to have the surface smoothness of 1100 sec. or more in terms of Bekk's smoothness, the fogging of the background is caused by the pressure applied during the surface treatment. Therefore, it is necessary that the base paper for the thermosensitive coloring layer be surface-treated beforehand to have a surface smoothness of 500 sec. or more, to prevent the fogging of the background. Japanese Laid-Open Patent Application No. 55-156086 discloses a thermosensitive recording material comprising a thermosensitive coloring layer with an optical surface smoothness (Ra) of 1.2 .mu.m or less and a glossiness of 25% or less.
In the above-mentioned prior art, only calendering methods such as supercalendering, machine calendering and gloss calendering are conducted in an attempt to improve the smoothness of the thermosensitive coloring layer. The calendering is applied only to the base paper, to both the base paper and thermosensitive recording paper, or only to the thermosensitive recording paper. When the surface smoothness and the image density of the thermosensitive recording paper are improved by the above-mentioned calendering, both a sticking problem and a piling problem occur. The sticking problem is such a problem that a thermal head sticks or adheres to the thermosensitive coloring layer and a noise is generated when the thermal head is separated from the thermosensitive coloring layer, or the dot reproduction property of the thermosensitive recording material is caused to deteriorate. The piling problem is such a problem that the heat-fused materials from the thermosensitive coloring layer are piled on the thermal head and cause the deterioration of image density and dot reproduction property. Because of those sticking and piling problems, the surface smoothness of the thermosensitive coloring layer is restricted to a level at which the image density, the sticking and the piling of the heat-fused materials are appropriately balanced in the conventional image formation. However, even though the smoothness of the thermosensitive recording materials is set at any level, the thermosensitive recording material is not satisfactory for high-speed recording in terms of image density and image stability.
The calendering methods have another shortcoming that the fogging of the background is caused by the pressure applied thereto during the calendering process, so that a clear distinction between a printed portion and the background is not obtainable. For instance, the image density is not always improved when the thermosensitive recording material is caused to have a surface smoothness of 1000 sec. or more in terms of Bekk's smoothness as a result of the supercalendering. In some cases, when such a calendering is performed, the slight uneveness of the base paper with respect to the basis weight thereof is intensified so that the head-matching property of the thermosensitive recording material is caused to deteriorate and the image density decreases.
When the surface smoothness of the thermosensitive recording material is improved by the above-mentioned calendering applied to the base paper or to the thermosensitive paper, the thickness of the thermosensitive recording material is reduced as a whole and the densities of the thermosensitive coloring layer and the base paper are increased. When this takes places, the voidages of the thermosensitive coloring layer and the base paper which is in contact therewith are reduced. Although this results in a slight improvement of the thermal conductivity of the thermosensitive coloring layer, it prevents the heat-fused materials on the surface of the thermosensitive coloring layer from penetrating into the inside of the thermosensitive recording material through the thermosensitive coloring layer and the base paper thereof during the recording step. It is considered that the heat-fused materials remaining on the surface of the thermosensitive coloring layer cause both the sticking and piling problems.
As stated above, when it is attempted to improve the image density by surface-treating the thermosensitive coloring layer with the calendering, the calendering inevitably causes the sticking and piling problems, and the fogging of the background. Therefore, it is difficult to improve the image density and also to prevent the sticking and piling problems and the fogging of the background.
Japanese Patent Application No. 58-136492 discloses a thermosensitive recording material comprising a lightly coated layer having an optical surface smoothness (Rp Value) of 8 .mu.m or less and a thickness of 40 .mu.m to 75 .mu.m. However, this thermosensitive recording material is not satisfactory in terms of thermal coloring sensitivity and dot reproduction property.
In order to prevent the fogging of the background and to enhance the dynamic coloring sensitivity of the recording material, several methods described below have been proposed. For instance, a material having a thermal conductivity of 0.04 Kcal/mh.degree. C. or less is employed as a support as disclosed in Japanese Laid-Open Patent Application 55-164192; and a layer comprising minute void particles as the main component is formed on a support as disclosed in Japanese Laid-Open Patent Applications 59-5093 and 59-225987. However, these thermosensitive recording materials prepared in the above manners are not satisfactory because they have low flexibility, poor heat insulating properties, and low thermal head-matching property.
Japanese Laid-Open Patent Application 62-5886 discloses a thermosensitive recording material comprising an intermediate layer which contains non-expandable minute void particles made of a thermoplastic resin, having a diameter of 5 .mu.m or less and Japanese Laid-Open Patent Application No. 1-113282, an undercoat layer containing spherical-void plastic particles having an average particle diameter ranging from 0.20 to 1.5 .mu.m and a voidage of 40 to 90%. However, the use of such plastic spherical-void particles is not a decisive manner for preparing a thermosensitive recording material having high sensitivity.